There exists a group of women for whom the risk of breast cancer far exceeds that of the normal female population. These women share certain characteristics: a strong family history of breast cancer, age greater than 30 at first birth, previously biopsied benign breast disease--the most powerful determinants. The risk of cancer in certain of these women may approach 1% annually. We estimate that perhaps 10% of all women between the ages of 45 and 70 fall into this high risk group. In the search for methods to prevent breast cancer, interest continues to grow in the field of dietary influences and, most particularly, fat consumption as a possible causative agent in the development of this disease. Since fat intake can be modified by changes in diet, it seems most logical to attempt dietary intervention in women at risk for breast cancer. Women at particularly high risk are a most logical choice since a change in their outcome should be measurable given a reasonably manageable sample size. The Kaiser-Permanente Medical Care Program and its Health Services Research Center has, for some time, been interested and active in several projects involving dietary manipulation. In the MRFIT, they learned and applied many sound techniques to modify dietary fat intake. In their own weight reduction program, Freedom From Fat, staff nutritionists have applied these techniques successfully to over 800 obese men and women. The present application is a response to an RFA for a large scale dietary intervention trial of five years duration. This organization is particularly well-suited for this effort, both by its large number of eligible participants and by its demonstrated expertise in dietary intervention.